Spear I had dropped in river (7)
I believe the answer is:
trident
'spear' is the definition.
(trident is a kind of spear)
'i had dropped in river' is the wordplay.
'i had' becomes 'id' (I'd).
'dropped in' indicates putting letters inside.
'river' becomes 'trent' (I've seen this before).
'id' going within 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Three-pointed spear" , "Spear with three prongs" , "Red tint (anag.)" , "spear carried at sea" , "Nuclear weapon" .)